The study of $J/\psi$ photoproduction at low energies has consequences for the understanding of multiple aspects of nonperturbative QCD, ranging from mechanical properties of the proton to the binding inside nuclei and the existence of hidden-charm pentaquarks. Factorization of the photon-$c\overline{c}$ and nucleon dynamics or vector meson dominance are often invoked to justify these studies. Alternatively, open-charm intermediate states have been proposed as the dominant mechanism underlying $J/\psi$ photoproduction. As the latter violates this factorization, it is important to estimate the relevance of such contributions. We analyze the latest differential and integrated photoproduction cross sections from the GlueX and $J/\psi -007$ experiments. We show that the data can be adequately described by a small number of partial waves, which we parametrize with generic models enforcing low-energy unitarity. The results suggest a non-negligible contribution from open-charm intermediate states. Furthermore, most of the models present an elastic scattering length incompatible with previous extractions based on vector meson dominance and thus call into question its applicability to heavy mesons. Our results indicate a wide array of physics possibilities that are compatible with present data and need to be disentangled.

The next generation of electron-hadron facilities has the potential for significantly improving our understanding of exotic hadrons. The $XYZ$ states have not been seen in photon-induced reactions so far. Their observation in such processes would provide an independent confirmation of their existence and offer new insights into their internal structure. Based on the known experimental data and the well-established quarkonium and Regge phenomenology, we give estimates for the exclusive cross sections of several $XYZ$ states. For energies near threshold we expect cross sections of few nanobarns for the $Z_c(3900{)}^{+}$ and upwards of tens of nanobarns for the $X(3872)$, which are well within reach of new facilities.

The first CLAS12 experiments will provide high-precision data on inclusive electron scattering observables at a photon virtuality $Q^2$ ranging from 0.05 to $12{\mathrm{GeV}}^2$ and center-of-mass energies $W$ up to 4 GeV. In view of this endeavor, we present the modeling of the resonant contributions to the inclusive electron scattering observables. As input, we use the existing CLAS electrocoupling results obtained from exclusive meson electroproduction data off protons, and evaluate for the first time the resonant contributions based on the experimental results on the nucleon resonance electroexcitation. The uncertainties are given by the data and duly propagated through a Monte Carlo approach. In this way, we obtain estimates for the resonant contributions, important for insight into the nucleon parton distributions in the resonance region and for the studies of quark-hadron duality.

The electroexcitation amplitudes or ${\gamma }_{v}p{N}^{*}$ electrocouplings of the $N\left(1440\right)1/2^{+}$, $N\left(1520\right)3/2^{-}$, and $\mathrm{\Delta }\left(1600\right)3/2^{+}$ resonances were obtained for the first time from the $ep\to e^{\prime }{\pi }^{+}{\pi }^{-}{p}^{\prime }$ differential cross sections measured with the CLAS detector at Jefferson Lab within the range of invariant mass $W=1.4\text{–}1.7$ GeV of the final state hadrons, for photon virtualities $Q^2=2.0\text{–}5.0$ ${\mathrm{GeV}}^2$. A good description of the nine independent onefold differential ${\gamma }_{v}p\to {\pi }^{+}{\pi }^{-}{p}^{\prime }$ cross sections achieved within the data-driven Jefferson Lab-Moscow State University (JM) meson-baryon reaction model in each bin of ($W,Q^2$) allows for separation of the resonant and nonresonant contributions. The electrocouplings were determined in the fits of the ${\pi }^{+}{\pi }^{-}p$ cross sections within three overlapping $W$ intervals with a substantial contribution from each of the three resonances listed above. Consistent results on the electrocouplings extracted from the data in these $W$ intervals provide evidence for their reliable extraction. These studies extend information on the electrocouplings of the $N\left(1440\right)1/2^{+}$ and $N\left(1520\right)3/2^{-}$ available from this channel over a broader range of $Q^2$. The electrocouplings of the $\mathrm{\Delta }\left(1600\right)3/2^{+}$, which decays preferentially into $\pi \pi N$ final states, have been determined for the first time. The reliable extraction of the electrocouplings for these states is also supported by the description of the ${\pi }^{+}{\pi }^{-}p$ differential cross sections with $Q^2$-independent masses and total/partial hadronic decay widths into the $\pi \mathrm{\Delta }$ and $\rho p$ final states.

Nucleon resonance contributions to the inclusive proton $F_2$ and $F_L$ structure functions are computed from resonance electroexcitation amplitudes in the mass range up to 1.75 GeV extracted from CLAS exclusive meson electroproduction data, taking into account interference between different excited nucleon states. The resonance contributions are compared with inclusive proton structure functions evaluated from $(e,e^{\prime }X)$ cross section data and the longitudinal to transverse cross section ratio. Contributions from isospin-1/2 and -3/2 resonances remain substantial over the entire range of photon virtualities $Q^2\lesssim 4{\mathrm{GeV}}^2$, where their electroexcitation amplitudes have been obtained, and their $Q^2$ evolution displays pronounced differences in the first, second, and third resonance regions. We compare the structure functions in the resonance region with those computed from parton distributions fitted to deep-inelastic scattering data, and extrapolated to the resonance region, providing new quantitative assessments of quark-hadron duality in inclusive electron-proton scattering.